1. Field of the Invention
The present invention relates to the technology field of three-phase power converters, and more particularly to a power converter with low common mode noise, which connect to a three-phase three-wire low-frequency alternating power source or a load.
2. Description of the Prior Art
Generally, three-phase power converter is applied in three-phase AC-DC systems, DC-AC systems, UPSs, wind power systems, and solar power systems for accomplishing AC-DC or DC-AC power conversion. Please refer to FIG. 1, which illustrate a diagram view of a typical topological structure of a three-phase power converter. As shown in FIG. 1, the typical three-phase power converter 1′ consists of: a power converting unit 11′, a filtering inductor set 12′, a filtering capacitor set 13′, and a line impedance stabilization network (LISN) 14′, where the midpoints (A′, B′, C′) of three arms of the power converting unit 11′ are taken to be three AC terminals, and coupled to three filtering inductors (LF1′, LF2′, LF3′) of the filtering inductor set 12′, respectively. Moreover, the three filtering inductors (LF1′, LF2′, LF3′) further coupled to three star-connected filtering capacitors Cx′ of the filtering capacitor set 13′. As shown in FIG. 1, one end of the three filtering capacitors Cx′ are coupled to three phase terminals (Ua′, Ub′, Uc′) of an external electrical grid 2′, and the other end of the three filtering capacitors Cx′ are connected to a netural point on the circuit. In addition, the LISN 14′ is coupled between the electrical grid 2′ and the filtering capacitor set 13′, and it is an auxiliary unit when tests conducted electromagnetic interference of the three-phase power converter 1′. Besides, three DC terminals (O′, P′, Q′) which formed by three DC filtering capacitors (CB0′, CB1′, CB2′) are taken as the output terminals of the power converting unit 11′, wherein the DC terminal O′ is the midpoint of the common DC bus, and both the DC filtering capacitor CB1′ and the DC filtering capacitor CB2′ connected to the DC terminal O′.
In the aforesaid three-phase power converter 1′, there is a stray capacitor C0′ between the ground GND′ and the common DC bus which connected with the three DC filtering capacitors (CB0′, CB1′, CB2′), and there are three stray capacitors (C1A′, C1B′, C1c′) between the ground GND′ and the midpoints (A′, B′, C′) of each of arms of the power converting unit 11′. Therefore, when the power converting unit 11′ is executed for voltage potential jumping, the distribution capacitors (C1A′, C1B′, C1C′) would produce corresponding displacement currents, and then the displacement currents further flow into the ground GND′, forming a common mode noise (common mode current) due to the three-phase power converter 1′.
Accordingly, an alternative three-phase power converter 1′ shown by FIG. 2 is proposed for solving the common mode noise issue. As shown in FIG. 2, a common mode passive filter 15′ is coupled between the electrical grid 2′ and the filtering capacitor set 13′, wherein the common mode passive filter 15′ comprises a common mode inductor 151′ and a filtering capacitor set 152′ including three star-connected filtering capacitors CY′.
Although the alternative three-phase power converter 1′ shown in FIG. 2 can indeed solve the common mode noise problem, it is very difficult to design and integrate the common mode inductor set 151′ because of the huge volume and high cost of the common mode inductors LCM′. FIG. 3 shows AC sweep results including curve 1, curve 2 and curve 3, wherein curve 1 represents the voltage spectrum of the three-phase power converter 1′ shown by FIG. 1, and curve 2 represents the voltage spectrum of the three-phase power converter shown by FIG. 2. From FIG. 3, it can find that, in spite of disposing the common mode passive filter 15′ between the electrical grid 2′ and the filtering capacitor set 13′ of the three-phase power converter 1′, there still has a large noise occurring near the resonant frequency f0 of the power converter 1′; that means the common mode passive filter 15′ cannot effectively suppress the common mode noise of the three-phase power converter 1′.
Accordingly, in view of the disposing the common mode passive filter 15′ cannot effective suppress the common mode noise of the three-phase power converter 1′, one point is that the inventor of the present application has made great efforts to make inventive research thereon and eventually provided a power converter with low common mode noise.